Encoders are used to measure angular or linear displacement. Optical encoders include photodiode arrays arranged in a detection plane for detecting incremental signals indicative of relative movement, e.g., rotary or linear movement, of encoder parts. Optical encoders include a light source, a scale index member (such as, e.g., a slotted or graduated code disk), and an array of photodiodes. A graduated code disk includes alternating transparent and opaque areas arranged periodically in a defined, fixed code disk pitch. In rotary encoders, the code disk is typically coupled to a rotating shaft and moved relative to the light source and the photodiode array.
Light is directed from the light source to the photodiode array by the scale index member or code disk. As the code disk rotates, in the case of an angular encoder, or moves linearly, in the case of a linear encoder, output signals from the photodiode array change. These varying output signals are used to measure angular or linear displacement.
An encoder's resolution refers to how sensitive the encoder is to displacement and depends on the amount of displacement between discrete measurement points. An encoder with a higher resolution allows more precise displacement measurement than encoders with lower resolutions. Increasing resolution may be complicated by manufacturing limitations, such as, e.g., a minimum width of the individual photodiodes and a minimum distance between adjacent, i.e., immediately adjacent, photodiodes. For example, under certain manufacturing constraints, it may not be possible to manufacture diodes having a pitch below, e.g., 9 μm
U.K. Published Patent Application 2 353 810 relates to a displacement measuring apparatus. To accommodate more closely packed photodiodes—and thus increase resolution—the photodiodes are deposited onto a substrate in two or more separate layers with adjacent photodiodes being deposited in different layers. This process may allow adjacent photodiodes to be more closely spaced than if the photodiodes were all deposited in the same layer. However, manufacture of such an apparatus requires complex sensor fabrication techniques, additional layers and interconnects, thereby resulting in increased manufacturing costs.